Patent Literature 1, for example, proposes adding an equivalent or larger amount of ammonia for reaction with nitrogen oxides in a combustion exhaust gas containing nitrogen oxides and sulfur oxides produced when coal is burned, as a reducing agent, to the combustion exhaust gas, thereby reducing the nitrogen oxides in the combustion exhaust gas in the presence of a denitration catalyst. Consequently, the combustion exhaust gas containing unreacted ammonia is introduced to a wet desulfurization apparatus on the downstream side of the denitration catalyst. The patent literature describes that this unreacted ammonia serves as a desulfurizing auxiliary agent, and therefore, the desulfuration rate of the combustion exhaust gas can be improved. In the technique described in Patent Literature 1, however, no consideration is given to removing mercury in the combustion exhaust gas.
On the other hand, a technique to remove mercury in a combustion exhaust gas is proposed in Patent Literature 2. This patent literature proposes adding ammonia to a combustion exhaust gas containing metallic mercury and hydrogen chloride exhausted from a coal-fired boiler to reduce nitrogen oxides in the presence of a denitration catalyst and oxidize highly-volatile metallic mercury to mercury chloride, removing burned ash in the combustion exhaust gas by an electric dust collector, and then allowing the mercury chloride and sulfur oxides to be absorbed into an absorbing solution of an wet desulfurization apparatus, thereby removing the mercury chloride from the combustion exhaust gas along with calcium sulfate.